1. Field of the Invention
This invention relates to an electromechanical filter and, more particularly, to a high performance mechanical frequency filter having low attenuation deviation in the pass band frequency range.
Mechanical filters comprising mechanical resonant elements which are mechanically connected together by coupling elements are now commonly used to produce certain filter frequency characteristics. Such mechanical filters are advantageous in commercial applications because of their small size and structure.
Recently, mechanical filters have been used as channel filters for carrier transmission and channel translating equipment. In this equipment, there is need for high performance mechanical filters having minimum attenuation deviation in the pass band frequency range. To achieve these stringent filter characteristics, the dimensional requirements of the mechanical resonator and coupling elements of the filter likewise become critical as will be described in greater detail hereinafter. In formulating a high performance mechanical channel filter, furthermore, the overall structure must be held to a minimum size.
All high performance mechanical filters, of course, must be highly reliable and manufactured in the most cost-effective way while ensuring the necessary filter characteristics. Two key factors in reducing cost is to provide a mechanical filter that is simple in structure and that has filter elements which are standardized in dimension and which do not require close dimensional tolerances.
2. Description of the Prior Art
To generally explain the characteristics of electromechanical filters, reference is made to FIG. 1(A) illustrating known elements of one type of mechanical filter and FIG. 1(B) showing an electrical circuit equivalent to the mechanical filter of FIG. 1(A).
As best seen in FIG. 1(A), a pair of resonators 1 and 3 are mechanically connected together by a coupler 2. The diameters of the resonators 1 and 3 are designated Dr, the lengths of the resonators 1 and 3 as L, the diameter of the coupler 2 as Dc, the length of the coupler 2 as Lc, the distance between the resonators 1 and 3 as Ls, and the distance from the edge of the torsional resonators to the coupler as x. In the filter illustrated, resonators 1 and 3 are considered to operate with torsional resonance characteristics while coupler 2 operates with a longitudinal resonance characteristic.
A mechanical filter, such as illustrated in FIG. 1(A), can be translated into an equivalent electrical circuit, and vice versa. The equivalent electrical circuit of the filter of FIG. 1(A) is depicted in FIG. 1(B). A series resonant circuit section 1' comprising elements L.sub.2 and C.sub.2 and a series resonant circuit section 3' comprising elements L.sub.1 and C.sub.1 correspond respectively to the mechanical resonators 1 and 3. A circuit section 2' with capacitor element C.sub.c corresponds to the mechanical coupler 2. Since the mechanical filter has inputs whereby an input electric signal is transformed into a mechanical vibration and outputs whereby a mechanical vibration is converted into an output electrical signal, the electrical circuit of FIG. 1(B) has corresponding input terminals 4 and 4' and output terminals 5 and 5'.
To achieve certain filter characteristics in commercial applications of mechanical filters, a series of mechanical resonators and couplers, such as shown in FIG. 1(A), are cascaded in succession. An equivalent electrical circuit is effected, accordingly, in the form of a chain circuit.
In the mechanical filters having a series of mechanical resonators, particularly those of pole-type configuration, there is frequently employed additional lines or "bridging couplers" in order to improve group delay time characteristics of the filter. Bridging couplers mechanically couple together non-successive resonators of a frequency filter, i.e., coupling lines which mechanically couple resonators that do not immediately follow each other. The equivalent electrical circuit for such a filter would include a capacitor, corresponding to the bridging coupler, connected in parallel with the equivalent electrical circuit in the chain circuit. Illustrative of an electromechanical filter having bridging couplers is shown, for example, in German patent Auslegeschrift No. 1,257,993.
To obtain desired filter characteristics by a mechanical filter, it is necessary to first develop the equivalent electrical circuit. After determining the values of the circuit components, such as the coils L.sub.1 and L.sub.2 and capacitors C.sub.1 and C.sub.2 in the series sections 3' and 1' and the capacitor C.sub.c in the parallel section 2' of the FIG. 1(B) circuit, the electrical values must be converted to corresponding mechanical resonators and coupling elements. Significantly, any variation in the capacitor C.sub.c of the parallel arm of FIG. 1(B), and its equivalent of the mechanical coupler 2, has a major influence on the desired filter characteristics. A highly accurate correspondence, therefore, must be made between these two components. To achieve this correspondence, it is neccesary to match what is commonly referred to in the art as the coupling factor or coefficient obtained from the electrical circuit with the mechanical configuration.
The coupling factor of a coupler, hereinafter designated as k, is best explained below with further reference to FIG. 1(B). If two resonant frequencies obtained at the terminals 4 and 4' when the terminals 5 and 5' being short-circuited fr.sub.1 and fr.sub.2 and fr.sub.1 &lt;fr.sub.2, the coupling factor k is expressed as follows: EQU k=(fr.sub.2 -fr.sub.1)/fr.sub.1
Wherein ##EQU1## In determining the capacitor in parallel section 2' of the electrical circuit of FIG. 1(B), the coupling factor k is a constant.
Difficulties are presented in selecting the length of a coupler to achieve the desired coupling factor k. Abnormal points occur in the coupling factor due to different mode oscillation or a bending mode resonance generated in the coupler, even though the coupler described previously has a longitudinal resonance characteristic so as to transfer the torsional resonance of a resonator or other resonators coupled. The abnormal points of the coupling factor can be determined depending upon the diameter Dc and length Lc of a coupler.
If, for the mechanical filter shown in FIG. 1(A), the diameters Dr of the resonators 1 and 3 are 3.0 mm.phi., the diameter Dc of the coupler 2 is 0.28 mm.phi., and the coupling position x is 2.50 mm, the relationship of the coupling length Lc to the coupling factor k is shown in FIG. 2. The abscissa of the FIG. 2 graph indicates length Lc (mm) of the coupler, while the ordinate indicates the coupling factor k (%). As can be seen in FIG. 2, the abnormal points of the coupling factor k are generated at designated points A, B, C, and D. Point A is at approximately the coupler length Lc of 3.3 (mm), point B at 5.3 (mm), point C at 7.3 (mm), and point D at 9.3 (mm). These abnormal points A, B, C, and D respectively represent different oscillation modes of the bending mode resonance characteristic of the coupler. For example, at point A, there is a first oscillation mode for the coupler, at point B a second oscillation mode for the coupler, at point C a third oscillation mode for the coupler, and at point D a fourth oscillation mode for the coupler. These modes are schematically illustrated in FIG. 2.
For use as a channel filter in channel translating equipment, a pole-type filter is recommended with the minimum possible number of components due to the requirement of group delay time characteristics for the pass band frequency range. In providing such a pole-type mechanical filter, the coupling factors for the couplers which mechanically connect the resonators become considerably different. Accordingly, various steps are necessary to prevent occurrence of bending mode oscillation of each coupler between resonators and avoid abnormal points in obtaining required coupling factors. To ensure the miniaturization of the channel filter, the coupler length is selected to be less than the second oscillation mode of the bending mode resonance characteristic of the coupler.
When using a cylindrical torsional resonator as shown in FIG. 1(A), the coupler length Lc is expressed approximately as: EQU Lc=Dr+Ls.
As can be seen from this equation and from FIG. 1(A), a coupler length Lc longer than Dr is required. The range of coupler lengths from which to select each of the required couplers is narrow when the coupler length is below the second oscillation mode of the bending mode resonance characteristic. Due to this limitation, it becomes difficult to obtain all of the desired coupling factors by use of a series of couplers betweens resonators which are formed in a single coupling wire. Various efforts have been made to achieve series coupling by a single coupling wire, but they do not prove completely satisfactory. For example, it is known to vary the diameters of the couplers as described in Japanese unexamined publication No. 52-56841, to change the connecting positions as described in Japanese examined publication No. 44-17402, and to change the diameters of the torsional resonators as described in Japanese unexamined publication No. 52-16946.
When, however, diameters of the couplers or the connecting positions are changed, use of different types of couplers become necessary and the number of coupler connecting points between the resonators increases. Moreover, when one or more of the torsional resonators are changed diametrically, use of different types of resonators also becomes necessary and the ability to connect couplers to the resonators, such as by spot welding, more difficult. In summary, material and manufacturing control become unduly complicated due to lack of standardization in the filter components. Automated manufacturing and mass production of such filters is difficult and expensive.
Further difficulties are encountered in manufacturing these filters because each coupler length is selected to be under the second oscillation mode of the bending mode resonance characteristic of the coupler. As best seen in FIG. 2, there is a large variation in the coupling factor k in relationship to a change in the coupler length Lc when the coupler length Lc is small. Consequently, tolerance of the coupler length must be held within close dimensions in manufacturing to obtain a desired coupling factor. To meet the stringent requirements in pass band frequency characteristics for a channel filter, error in the coupling factors should be held to within one percent (1%). This compounds the problem. In order to maintain an error in the coupling factor of within one percent while using a coupler length shorter than the second oscillation mode of the bending mode resonance characteristic, tolerance in the coupler length is extremely tight.
For the type of mechanical filter illustrated in FIG. 1(A), when the coupler length Lc is below the first oscillation mode as shown in FIG. 2, tolerance in the coupler length must be held to approximately 30 .mu.m. When the length of the coupler is between the first and second oscillation modes, the tolerance in the coupler length is approximately 60 .mu.m. For a coupler length between the second and third oscillation modes, the tolerance is approximately 120 .mu.m.
As can be seen from the foregoing remarks, electromechanical filters with minimum attenuation deviation in the pass band frequency range and which are simple in structure, reliable, and easily manufactured, have yet to be satisfactorily achieved.